Coordinate measuring machine (CMM) and method of compensating errors in a CMM

ABSTRACT

A coordinate measuring machine for determination of at least one spatial coordinate of a measurement point on an object to be measured. The coordinate measuring machine comprises a stationary base, a probe head for approaching the measurement point and a frame structure for linking the probe head to the base. A first reference path is provided by an optical reference beam, wherein the reference beam extends along a guide of a linear drive mechanism so that the reference path is parallel to a first direction. Furthermore, at least one displacement sensor is assigned to the reference beam, the reference beam and the displacement sensor being designed and arranged in such a way, that a displacement of the movable member of the linear drive mechanism relative to the first reference path is measurable being indicative of a translational and/or rotational displacement of the movable member from its ordinary bearing position.

FIELD OF THE INVENTION

The present invention relates generally to a coordinate measuringmachine (CMM) for determination of at least one space coordinate of ameasurement point and to a method of compensating errors in a coordinatemeasuring machine (CMM) as set forth in the claims.

BACKGROUND

It is common practice after workpieces have been produced to inspectthem on a coordinate positioning apparatus, such as a coordinatemeasuring machine (CMM) having a movable probe head within a workingvolume of the machine.

In a conventional three-dimensional measuring machine, the probe head issupported for movement along three mutually perpendicular axes (indirections X, Y and Z).

In a simple form of the machine a suitable transducer mounted parallelto each axis is able to determine the position of the probe headrelative to a base of the machine and, therefore, to determine thecoordinates of a measurement point on an object being approached by theprobe.

There are several possible sources of error if such a technique isemployed. Lack of straightness in movement and of orthogonality of theaxes is one major cause of such errors. A further cause of error is theangular rotation of the carriages about axes perpendicular to theirdirections of movement. Such errors, often referred to as Abbé errors,depend not only upon rotation but also upon a lateral offset in thelinear drive mechanisms.

Further sources of errors may include external influences likevibrations, temperature variation, pressure variation, humidityvariation, aging of components of the CMM-frame-structure, etc.

Particularly, the following error factors may occur:

-   -   scale errors on axes,    -   horizontal straightness errors on axes,    -   vertical straightness errors on axes,    -   pitching errors on axes,    -   yawing errors on axes,    -   rolling errors on axes, and    -   angular errors between axes.

Also, weaknesses in the frame structure of the CMM—which may lead to abending of e.g. the legs or the bridge of the CMM—cause errors.

Many attempts have been made to provide correction for the varioussources of error referred to. For example, it is known to introduce adeliberate and known error into the transducers by various means.However, such corrections only apply for a given location in themeasuring volume.

An alternative technique is to calibrate the machine, measuring theerrors existing at various points and storing these so that they may beapplied when the machine is actually used. Exemplarily in connectionwith such calibration methods, for each axis (x,y,z) and each lineardrive mechanism, some axis dependent geometrical errors aremeasured—e.g. pitch, yaw, straightness (in two orthogonal axis) androll. That measurement can be done by some kind of increment (forexample every 20 mm), the gathered data is stored in a calibration table(in the machine or the software) and is used when running the machine insuch a way, that the data in the table corrects the geometrical errorsdepending on the position. In addition to this there is of course theangularity between the axis (x,y,z) and some scale factor for each axis.The calibration method is usually carried out on a complete assembledmachine.

As may be imagined, such a calibration process is lengthy, especiallyfor a large machine. However, any “settling” of the machine during usewould invalidate the calibrations. Another drawback with the calibrationmethods is that they will only take care of fully repeatable errors. Itis also necessary to calibrate the probe during the same conditions asin the working state of the machine. This means that if the machine runswith 100 mm/sec, the calibration procedure also should be performed withthat speed, and if—by some reason—a change of the running speed isnecessary, a recalibration of the machine with this new speed would berequired.

Another aspect which has to be considered is that accelerations of theprobe cause dynamic deflections of the coordinate measuring machinewhich in turn cause measurement errors. These measurement errors may bereduced by taking measurements at low accelerations. However,productivity demands an increased throughput as well as an increasedinspection speed. Hence, the probe experiences higher accelerationsduring the measurements and larger dynamic structural deflections of thesystem—in particular the frame structure of the CMM—result. This causesinaccurate reporting of the X,Y,Z geometric position of the probe.

In particular, some coordinate measuring machines exhibit significantdrive vibration at high speed. One source of error causing the vibrationis the machine mechanical drive system. Errors caused by thesevibrations (typically above 5 Hz) are not suitable for calculativemethods of compensation for dynamic errors described above as thevibration causes non repeatable behaviour at high speed which causesmeasurement errors.

Furthermore, a variety of probes are employed in a coordinate measuringmachine for measurements within the scale coordinate system, whichincludes reference scales arranged along axes that configure thethree-dimensional measuring space. To provide the coordinate measuringmachine with an improved measurement precision, the structure of theframe thereof is required to have a high static stiffness.

Exemplary, EP 1 559 990 discloses a coordinate measuring system andmethod of correcting coordinates measured in a coordinate measuringmachine. Thereby, geometrical errors are measured while works withvarious weights are mounted on the coordinate measuring machine.Compensation parameters are derived from measured results per a weightof a work and stored. A compensation parameter corresponding to a weightof a work to be measured is appropriately read out to correct measuredcoordinates of the work to be measured.

As a further example, EP 1 687 589 discloses a method of errorcompensation in a coordinate measuring machine with an articulatingprobe head having a surface detecting device. The surface detectingdevice is rotated about at least one axis of the articulating probe headduring measurement. The method comprises the steps of: determining thestiffness of the whole or part of the apparatus, determining one or morefactors which relate to the load applied by the articulating probe headat any particular instant, and determining the measurement error at thesurface sensing device caused by the load.

Also, GB 2 042 719 discloses a measuring apparatus having three mutuallyperpendicular axes, wherein errors due to rotations about the variousaxes are corrected.

Another approach for error correction of work piece measurements with acoordinate measuring machine (CMM) is disclosed in GB 2 425 840.Thereby, position measurements are taken with a work piece sensingprobe, in which means of measuring acceleration are provided. Themeasurements are corrected for both high frequency (unrepeatable) errorssuch as those due to vibration, and low frequency (repeatable) errorssuch as those due to centrifugal forces on the probe. The correctionmethod comprises measuring the work piece, determining repeatablemeasurement errors from a predetermined error function, error map orerror look-up table, measuring acceleration and calculating unrepeatablemeasurement errors, combining the first and second measurement errors todetermine total errors and correcting the work piece measurements usingthe total errors. The predetermined error map is calculated using anartefact of known dimensions.

It is also known to use accelerometers fitted in the probe (or Z-column)of the machine and in the base table (for a differential measurement).The displacements and errors of the probe-position are measured withdouble integration, and from that it will be possible to adjust thereading with the difference between the double integrated signal and thescales.

However, when using accelerometers, they will usually become noisy whenthe frequency is relatively low. This can give a bad signal to noiseratio. Furthermore, it may only be possible to measure differencesduring acceleration, which means that—in general—it may be necessary tocalculate the acceleration from the scale position and to compare itwith the measured acceleration, and double integrate the difference.However, this may not be enough information to accurately calculate theexact position of the probe. Using such a method also doesn't allowmeasuring static changes (i.e. friction in combination with dynamicchanges will not be considered).

SUMMARY

It is therefore an object of the present invention to provide animproved coordinate measuring machine CMM and method, wherein errorscaused by dynamic affects (e.g. when running a high speed scanning),errors caused by week structures or static changes (e.g. changesintroduced by friction or load onto the frame structure of the CMM)and/or errors caused by external influences (e.g. temperaturevariations, vibration, pressure) can be compensated for in an improvedmanner.

In particular, displacement errors in each linear drive mechanism (inthe X,Y,Z directions) of the CMM and/or deflections and deformations(e.g. bending) in the frame structure of the CMM caused e.g. by theload, the movements and/or the accelerations of the probe should berecognized and compensated for precisely.

This object is achieved by realising the features of the independentclaims. Features which further develop the invention in an alternativeor advantageous manner are described in the dependent patent claims.

The present invention relates to a coordinate measuring machine (CMM)for determination of at least one spatial coordinate of a measurementpoint on an object to be measured. The CMM comprises at least a base(particularly a stationary base, e.g. a measurement table for supportingthe object to be measured), a probe head for approaching the measurementpoint and a frame structure for linking the probe head to the base.

The frame structure comprises at least a first and a second framecomponent and at least one linear drive mechanism moveably linking thefirst and the second frame components in such a way, that the probe headis movable relative to the base in at least a first direction (X,Y,Z).

Therein, the at least one linear drive mechanism comprises a linearguide in the first direction, a movable member being supported formovement along the guide by bearings, and a linear measuring instrumentfor determination of a first drive position of the movable member in thefirst direction (X,Y,Z).

According to the invention, a first reference path in provided by atleast a first optical reference element designed as an optical referencebeam, wherein the reference beam extends along the guide of the lineardrive mechanism so that the reference path is parallel to the firstdirection (X,Y,Z).

Furthermore, at least one displacement sensor is assigned to thereference beam, the reference beam and the displacement sensor beingdesigned and arranged in such a way, that a displacement of the movablemember relative to the first reference path is measurable beingindicative of a translational and/or rotational displacement of themovable member from an ordinary bearing position.

Particularly, the displacement sensor can comprise a photosensitivedetector element being built for measuring a distance to the referencebeam and/or an impinging position of the reference beam, the distancerespectively the impinging position being indicative of the displacementof the movable member relative to the first reference path in adirection perpendicular to the first direction (X,Y,Z).

In more detail, the displacement sensor may further comprise a beamsplitter for coupling out at least a part of the reference beam anddirecting it onto the photosensitive detector element. Therein, thephoto-sensitive detector element may exemplarily be built as CCD-array,CMOS-array, PSD or quadrant detector.

In case that—exemplarily—the linear guide is provided on or by the firstframe component and the movable member is provided on or by the secondframe component, a laser source may be installed on the first framecomponent providing for the reference beam and the at least onedisplacement sensor can be attached to the second frame component insuch a way that it faces towards the laser source.

Therein, the laser source may for example be designed as a laser diodewith collimation optics.

According to a further development of the invention, also two or moredisplacement sensors—particularly three to five—can be assigned to thereference beam, the reference beam and the displacement sensors beingdesigned and arranged in such a way, that two or more distances fromdefined positions on the movable member to the reference beam and/orimpinging positions of the reference beam are measurable by thedisplacement sensors, wherein the design is constructed so that thedistances respectively impinging positions indicate translational androtational displacements of the movable member from an ordinary bearingposition.

The coordinate measuring machine (CMM) further comprises a calculationunit for determination of the space coordinate. According to theinvention, not only the linear positions of the drive mechanisms (e.g.the first drive position of the first linear drive mechanism), but alsothe measured translational and/or rotational displacements of themovable member are considered for determination of the space coordinate.

Hence, the determination of the space coordinates is carried out as afunction of at least

-   -   the first drive position of the first linear drive mechanism and    -   the translational and/or rotational displacements of the movable        member from the ordinary bearing position.

According to the methodological aspects of the invention, the followingsteps are performed:

-   -   providing a first reference path parallel to the first direction        (X,Y,Z) by generating an optical reference beam extending along        the guide of the linear drive mechanism,    -   measuring at least one displacement of the movable member        relative to the first reference path so that the at least one        displacement is indicative of a translational and/or rotational        displacement of the movable member from an ordinary bearing        position, and    -   compensating errors occurring in connection with measurements        performed by the inventive and above described CMM, particularly        weaknesses in the bearing of the linear drive mechanism, by        using the determined actual at least one displacement.

According to the invention, there will be no need (at least notnecessarily) to carry out a separate and lengthy calibration procedureof the axis dependent geometrical errors on an assembled machine inadvance—as known from the state of the art.

Hence, because the axis dependent geometrical errors can be sensed inparallel and concurrent to real measurements, the disadvantages ofcompensating for errors by performing a calibration method according tothe state of the art (i.e. time extensive calibration process; differentconditions invalidate the calibrations; only fully repeatable errors canbe considered; etc.) can be removed or at least reduced significantlyaccording to the invention.

According to further aspects of the CMM of the invention, not only anoptical reference beam (i.e. the first optical reference element) may beprovided, but also a second or more reference elements may be arrangedon the frame structure, each for providing a substantially unloadedreference path along a part of the frame structure, wherein at least onedisplacement sensor, in particular two to five displacement sensors, canbe assigned to each of the reference elements. According to a moregeneral scope, the reference elements and the displacement sensors arethen designed and arranged in such a way, that displacements and/ordeformations of the frame structure are measurable relative to theunloaded reference paths.

Therein, the second or more reference elements may either be designed asfurther optical reference beams analogous to the first optical referenceelement as described above. Alternatively, however, one or more of thesecond or more reference elements may also be designed as a mechanicalreference element extending along a part of the frame structure, whereinthe mechanical reference element can be fastened fixedly to the framestructure in a substantially unloaded way.

Regarding both optical and mechanical alternatives, as a more generalaspect, the second or more reference elements can be arranged on theframe structure for providing a substantially unloaded reference pathalong a part of the frame structure.

Furthermore, at least one displacement sensor is assigned to thereference elements, wherein the reference elements and the displacementsensor being designed and arranged in such a way, that displacementsand/or deformations of the frame structure in the region of therespective parts are measurable relative to the reference paths.

Particularly, the reference elements and the displacement sensors aredesigned and arranged in such a way, that a distances between thereference paths and defined locations on the frame structure aremeasurable by the displacement sensors, wherein the distances indicatethe displacements and/or deformations of the frame structure in theregion of the respective parts.

In general, the reference element may extend over the linear drivemechanism and at least a part of one of the frame components. Therein,the reference element is fixedly fastened to the first frame componentand the displacement sensor is arranged in such a way that a distancefrom the reference element to a defined location on the frame componentis measurable. For example, the mechanical reference element may beinstalled in such a way that it elongates along the linear drivemechanism parallel to the linear moving direction of the movable member,wherein the reference element is mounted in a way decoupled from forcesof the frame structure of the CMM. One or more displacement sensors canthen be arranged so as to detect a distance between a defined locationof the movable member and the mechanical reference element. Thisdistance may indicate a translational displacement of the movable memberfrom an ordinary bearing position (in case of more than one measureddistance, also rotational displacements can be indicated).

As known per se from the state of the art, the linear drive mechanismcan comprise a linear guide in the first direction, a movable memberbeing supported for movement along the guide by bearings, and a linearmeasuring instrument for determination of a first drive position of themovable member in the first direction (X,Y,Z). Furthermore, thecoordinate measuring machine may comprise a calculation unit fordetermination of the space coordinate of the point to be measured (andbeing approached by the probe) as a function of at least the first driveposition. According to the invention, also the detected deformationand/or the displacements are considered for the determination of thespace coordinate. For example, the sensed displacements and/ordeformations may directly be considered when calculating the spatialposition of the probe head with respect to a datum position and/or beused in order to compensate for scale errors in connection withdetermination of the travelling positions by the linear measuringinstruments.

Alternatively to the embodiment wherein the reference element extendsover a part of the CMM which comprises a drive mechanism, one of thereference elements may also be designed and arranged in such a way thatit extends only along a non-movable part of the frame structure (e.g.along at least a part of the legs or the bridge). The reference elementcan be installed e.g. parallel to and—in particular minimally—distancedapart from a surface of the frame structure. Particularly, the referenceelement can be fixed to the frame structure only on one of its ends andthe displacement sensor may be arranged in order to detect a distancebetween the other end of the reference element and a facing definedlocation on the frame structure. That means, only a first end of thereference element is fixedly attached to the frame structure and thedisplacement sensor is arranged in such a way, that the displacementand/or deformation is indicated by measuring a position of a definedlocation on the frame structure with respect to the facing second end ofthe reference element.

As mentioned, one of the second or more reference elements can bedesigned as an elongated mechanical reference element—e.g. a referenceframe or reference rod—extending along a first part of the framestructure. The reference frame or rod is fastened fixedly to the framestructure in a substantially unloaded way. Particularly, the referenceframe may be fastened only on one of its ends to the frame structure.The displacement sensor can be built as optical, capacitive or inductivedistance sensor and may be arranged either on the reference frame or atthe defined location on the frame structure. The sensed distanceindicates a deformation of a known part of the frame structure or adisplacement in the linear drive mechanism (e.g. a displacement of themovable member from its ordinary bearing position).

The mechanical reference element may be designed from material beinghighly resistant against deformation caused by temperature, pressure,humidity, aging or similar factors. Exemplarily, the mechanicalreference element (e.g. the reference rod) may consist of or compriseinvar or carbon fiber material.

Alternatively to a mechanical design of the reference element, accordingto the invention, the first reference element (and also one or more ofthe second or more reference elements) can also be designed as opticalreference beam, in particular a collimated or focused laser beam, whichextends along a known part of the frame structure. The reference beammay be emitted by a laser source mounted directly onto the framestructure on a known location and in a defined and known direction.Particularly, the laser beam in directed parallel to a surface of theCMM frame structure. Therein, the displacement sensor can built as aphotosensitive detector element being built for measuring a distance tothe reference beam and/or an impinging position of the reference beam.Particularly, the displacement sensor may further comprise a beamsplitter for coupling out at least a part of the reference beam anddirecting it onto the photosensitive detector element. Thephotosensitive detector element may be designed as a CCD-array,CMOS-array, PSD-sensor or quadrant detector.

Therein, the reference path is functionally generated or represented bythe reference element. In case that the reference element is designed asmechanical reference element, the reference path may e.g. be representedby the surface thereof, wherein the shape of the element may highprecisely be measured and calibrated before installing it onto the CMM.In case that the reference element is designed as optical referencebeam, the path may be represented by the optical axes of the beam.

Furthermore, according to the invention, also two or more displacementsensors, particularly three to five, can be assigned to each referenceelement, wherein the reference element and the displacement sensorsbeing designed and arranged in such a way, that two or more distancesbetween the first reference element (i.e. reference path) and definedlocations on the frame structure are measurable by the displacementsensors. The sensed distances, thus, can indicate displacements and/ordeformations of the frame structure in the region of the first part withmore than one degree of freedom.

According to the generic art of CMMs, preferably the frame structurecomprises four frame components and three linear drive mechanismsmoveably linking the four frame components, for provision of movabilityof the probe head relative to the base in a first, a second and a thirddirection X,Y,Z. As known per se, each linear drive mechanism maycomprise a linear guide in the first, the second respectively the thirddirection X,Y,Z, a movable member being supported for movement along theguide by bearings and a linear measuring instrument for determination ofthe first, a second or a third drive position, respectively, of themovable member in the first, the second or the third direction X,Y,Z,respectively. The calculation unit of the CMM can be designed fordetermination of the space coordinate as a function of at least thefirst, the second and the third drive position as well as—according tothe invention—the deformation and/or the displacement indicated by thedisplacement sensors.

Summed up, by measuring distances between defined locations on the framestructure and the reference elements/paths, displacements and/ordeflections of the frame structure as well as displacements of thetravelling members (carriages) can be sensed and determined. Asadvantage with respect to the state of the art, the method can be usedfor compensating static changes (changes introduced by friction, etc.)as well as dynamic effects (accelerations of the probe head).

Also a method of compensating errors in a coordinate measuring machineas described above is provided, the CMM determining at least one spatialcoordinate of a measurement point on an object to be measured. Accordingto the method, at least one displacement of the frame structure in theregion of a first part is measured relative to an external,substantially unloaded, reference element which extends along the firstpart of the frame structure and is arranged thereon. The errors,particularly weaknesses in a bearing of the linear drive mechanismand/or deformations in the material of the frame structure (caused bythe load of the probe and/or variations in temperature, pressure,humidity) are compensated by using at least the determined actualdisplacement and/or deformation.

Hence, there will be no need (at least not necessarily) to carry out aseparate and lengthy calibration procedure of the axis dependentgeometrical errors on an assembled machine in advance—as known from thestate of the art.

In case that the reference element is designed as an optical reference(collimated laser beam or similar), it can be assumed that the beam isstraight and the axis dependent geometrical errors can be measureddirectly by using the displacement sensors (which measure the deviationswith respect to the beam preferably in different directions). In casethat the reference element is designed as mechanical reference (e.g.physical beam) the reference beam may separately be measured andcalibrated before installing it onto the CMM. By doing so, the sameresults may be achieved as for the light beam, i.e. that the shape andcourse of the mechanical reference element is known with high precision.This means in general that the geometrical accuracy is located in thereference element and not in the machine structure.

Hence, because the axis dependent geometrical errors can be sensed inparallel and concurrent to real measurements, the disadvantages ofcompensating for errors by performing a calibration method according tothe state of the art (i.e. time extensive calibration process; differentconditions invalidate the calibrations; only fully repeatable errors canbe considered; etc.) can be removed or at least reduced significantlyaccording to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in greater detail below with referenceto examples of possible embodiments shown schematically in the drawings,in which:

FIG. 1 shows an—exemplarily bridge-type—coordinate measuring machineaccording to the invention;

FIG. 2 shows reference rods as mechanical reference elements beingmounted to the frame structure only on of its ends;

FIG. 3 shows a reference laser beam as optical reference element for thex-carriage;

FIG. 4 shows a linear X-drive mechanism, wherein a collimated referencelaser beam as reference element extends along the bridge and twodisplacement sensors are arranged on the X-carriage;

FIG. 5 shows optical displacement sensors, each for measuring a distancefrom the reference laser beam;

FIG. 6 shows a side view of a bridge-type CMM with a mechanicalreference element, wherein displacement sensors are assigned to thereference element;

FIG. 7 shows a side view of a bridge-type CMM with a mechanicalreference element, wherein displacement sensors are assigned to thereference element;

FIG. 8 shows a side view of a bridge-type CMM with a mechanicalreference element, wherein displacement sensors are assigned to thereference element;

FIG. 9 shows front view of a gantry-type CMM—according to theinvention—with a mechanical reference element and an optical referenceelement;

FIG. 10 shows a side view of the gantry-type CMM of FIG. 9;

FIG. 11 shows a front view of a bridge-type CMM with an opticalreference element for indicating a bending of the bridge;

FIG. 12 shows a front view of a bridge-type CMM with an opticalreference element for indicating a bending of the bridge; and

FIG. 13 shows a front view of a bridge-type CMM with an opticalreference element for indicating a bending of the bridge.

DETAILED DESCRIPTION

In FIG. 1, an exemplary embodiment of the coordinate measuring machine 1having a frame structure 15 for linking the probe head 6 to the base 3(wherein the frame structure 15 comprises several frame components14,22,24 being movable with respect to one another) according to theinvention is depicted.

In detail, the coordinate measuring machine 1 comprises the base 3, onwhich a portal 14 (as one of the frame components)—being supported bybearings—is arranged so that it can be moved in the longitudinaldirection (Y-direction). The portal 14 has two portal legs 16,18, whichare connected by a bridge 20 at their upper ends.

An X-carriage 22 (as further frame component), which can be driven alongthe bridge, i.e. in a space direction connecting the two portal legs16,18 (X-direction), is placed on the bridge 20. A ram or Z-column 24(as further frame component) can be shifted in a third space direction(Z-direction). Therefore, the Z-column 24 is supported for movement inthe Z-direction by bearings which are integral with X-carriage 22. Thethree space directions X, Y and Z are preferably orthogonal to oneanother, although this is not necessary for the present invention.

Usually, the components of the frame structure of the CMM (i.e. theX-carriage, X-beam (bridge)) may be made of aluminium, granite, ceramicsor steel/iron and has wall-thickness and stiffness adapted to the load.

The two most common types of bearings between the movable members andthe guides are air bearings or mechanical bearings (e.g. linearcirculating plus rails). The air bearings give the advantage that thereis no friction in the movement (which may introduce different kind oferrors like angle errors or hysteresis). The disadvantage of airbearings is that the stiffness is normally lower than in mechanicalbearings, so that particularly dynamic errors may occur. In mechanicaltypes, the stiffness in the bearing system is normally higher but thereis friction and the friction forces may introduce errors. However, theinvention may be applied for both types of bearings.

Summed up, the coordinate measuring machine 1 is built for determinationof three space coordinates of a measurement point 13 on an object 12 tobe measured and, therefore, comprises three linear drive mechanisms forprovision of movability of the probe head 6 relative to the base 3 inthe first, second and third direction (X, Y and Z direction).

Each linear drive mechanism has a linear guide, one in the first, one inthe second and one in the third direction (X, Y and Z direction),respectively. In a simple embodiment, the linear guide of theY-direction drive mechanism is formed by two edge-building surfaces ofthe base 3, the linear guide of the X-direction drive mechanism isformed by two or three surfaces of the bridge 20 and the linear guide ofthe Z-direction drive mechanism is formed by a cubical hole in theX-carriage member.

Furthermore, each linear drive mechanism comprises a movable memberbeing supported for movement along the guide by bearings. In particular,the movable member of the X-direction drive mechanism is embodied asY-carriage 28 having mutually facing surfaces with respect to the abovementioned two guiding surfaces of the base 3. The movable member of theX-direction drive mechanism is embodied as X-carriage 22 having mutuallyfacing surfaces with respect to the above mentioned two or three guidingsurfaces of the bridge 20. And, the movable member of the Z-directiondrive mechanism is formed by Z-column 24 having mutually facing surfaceswith respect to the inner surfaces of the cubical hole in the X-carriage22.

Moreover, each linear drive mechanism comprises a linear measuringinstrument for determination of a first, a second or a third driveposition, respectively, of each movable member in the first, the secondor the third direction (X, Y and Z direction), respectively.

A probe head 6, on which a stylus is arranged exemplarily, is fastenedon the lower free end of the Z-column 24. The stylus is used in a mannerknown per se for touching the object 12 to be measured. However, thepresent invention is not restricted to a tactile coordinate measuringmachine and may likewise be used for coordinate measuring machines inwhich a measurement point is approached in a non-contact manner, i.e.for example a coordinate measuring machine with an optical scanninghead. More generally, the probe head 6 may be designed for arranging acontact probe, e.g. a scanning or touch trigger probe, or a non-contactprobe, particularly an optical, capacitance or inductance probe.

Furthermore, the invention is not restricted to a coordinate measuringmachine in the portal bridge design as shown here. It may equally beused for coordinate measuring machines in gantry design as depicted inFIGS. 8 and 9, in which only the bridge 20 with two supports,functioning as very short feet, can travel along two highly placed fixedrails. Moreover, the invention may generally be used for all types ofcoordinate measuring machines, i.e. for a CMM being designed asparallel-kinematics machine as well as for a CMM having linear or serialkinematics. Exemplarily, the CMM may be designed in bridge-type,L-bridge-type, horizontal-arm-type, cantilever-type or gantry-type.

In this exemplary embodiment of FIG. 1, the portal legs 16,18 each havea movable Y-carriage 28 which allow movement of the portal 14—includingbridge 20—in the Y-direction.

A measuring scale 30Y being part of the Y-measuring instrument isschematically represented on the long side of the base 3, wherein thescale 30Y extends parallel to the Y-direction. The scale may be a glassmeasuring scale, e.g. having incremental or absolute coding, with whicha drive position in the Y-direction of the Y-carriage 28 can bedetermined. It is to be understood that the measuring instrument mayfurthermore contain suitable sensors for reading the measuring scale30Y, although for the sake of simplicity these are not represented here.However, it should be pointed out that the invention is not restrictedto the use of glass measuring scales, and therefore may also be usedwith other measuring instruments for recording thedrive/travelling-positions of the movable members of the drivemechanisms.

Another measuring scale 30X is arranged parallel to the X-direction onthe bridge 20. Finally, another measuring scale 30Z is also arrangedparallel to the Z-direction on the Z-ram 24. By means of the measuringscales 30X,30Z as part of the linear measuring instruments, it ispossible to record the present drive positions of the X-carriage 22 inX-direction and of the Z-column 24 in the Z-direction metrologically ina manner which is known per se.

In the shown embodiment, the base 3 comprises a table with a granitesurface plate for supporting an object 12 to be measured, on which thespace coordinates of the measurement point 13 are intended to bedetermined.

Also shown is a control and calculation unit 11, which is designed toactuate the motor drives of the coordinate measuring machine 1 so thatthe probe head 6 travels to the measurement point 13. For manualoperation, the control unit 11 may be connected to a user console 32. Itis also possible for the control unit 11 to fully automatically approachand measure measurement points 13 of the object 12 to be measured.

The control and calculation unit 11 contains a processor 34 and aplurality of memories 36,38. In particular, the control and calculationunit 11 is designed for determining the three space-coordinates of themeasurement point 13 on the object 12 as a function of at least thefirst, the second and the third drive position of the three drivemechanisms.

According to the invention, as shown in more detail in the followingfigures, the reference element 71 may be installed in such a way that itelongates along the linear drive mechanism parallel to the linear movingdirection Y of the movable member, wherein the reference element 71 isunloaded and, therefore, decoupled from forces which are carried by theCMM frame structure. For example, the reference element 71 is designedas collimated reference laser beam emitted by a laser source 75. One ormore displacement sensors 9 a,9 b can then be arranged onto the carriageof the movable member so that they are able to detect a distance betweendefined locations of the movable member and the reference laser beam.These distances indicate a translational or rotational displacement ofthe movable member from an ordinary bearing position (e.g. atranslational displacement in X- and/or Z-direction or a pitchingerror).

The thereby sensed displacement or displacements, hence, may further beused, for example, in order to

-   -   directly compensate for scale errors in connection with        determination of the travelling positions by the linear        measuring instruments and/or    -   compensate for the sensed horizontal straightness errors,        vertical straightness errors, pitching errors, yawing errors        and/or rolling errors of the carriage (moving member) in        connection with the calculation of the position of the probe        head by the control and calculation unit 11 (i.e. when deriving        the space coordinate of the measuring point on the object to be        measured).

Because the design of coordinate measuring machines of the generic kindas well as the design of different linear guides and different linearmeasuring instruments are well known to skilled persons, it must beunderstood that numerous modifications and combinations of differentfeatures can be made. All of these modifications lie within the scope ofthe invention.

FIG. 2 shows two reference rods as mechanical reference elements 72 a,72b being mounted to the frame structure only on of its ends. Thereby, forsake of simplicity, only the bridge 20 (X-beam) with the X-carriage 22and the Z-beam 24 are depicted as CMM-components. The X-carriage 22 issupported for linear X-movement with respect to the bridge 20—whichitself serves as linear X-guide—by bearings. The Z-guide may be formedby a cubical hole in the X-carriage member 22, through which the Z-beam24 can be moved along the Z-axis.

The mechanical reference elements 72 a,72 b are preferably made from amaterial with a high dimensional stability and high solid gauge, so thatit is insusceptible to external influences as temperature, pressure,humidity, aging, etc.

In particular, the mechanical reference elements 72 a,72 b can befastened fixedly to the frame structure only on one of its ends in sucha way that it extends parallel along a part of the frame structure. Thismay allow that the reference element is mounted force-decoupled from theframe structure of the CMM, so that the reference path generated by thereference element is completely or at least substantially unloaded.

One displacement sensor 9 a is assigned to the first reference rod 72 a(which extends along the bridge) for measuring a distance from a definedposition of the carriage 22 to the reference path generated by the firstreference rod 72 a. The displacement sensor 9 a may be attached to thecarriage 22, so that the distance to the reference path can be measured.

For example, the sensed distance indicates a translational displacementof the carriage 22 from an ordinary bearing position (e.g. atranslational displacement in Z-direction of the carriage 22 relative tothe guiding surface of the X-beam 20).

Also, one other displacement sensor 9 b is assigned to the secondreference rod 72 b, wherein the second reference rod 72 b is fixedlyattached—on one of its ends—to the Z-beam 24. The second reference rod72 b extends parallel to and a defined distance apart from the Z-beam 72b, so that displacement-measurements relating to weaknesses in theZ-drive mechanism are independent from a load condition of the z-beam24. The displacement sensor 9 b, therefore, can be mounted onto theX-carriage 22 and face towards the second reference 72 b in order tomeasure a distance form the X-carriage 22 (functioning as guide for theZ-movement of the Z-beam 24) to the second reference rod 72 b. Thisdistance relates to an actual bearing distance in the Z-drive mechanismand, therefore, indicates a translational displacement from an ordinarybearing condition in the X-direction.

FIG. 3 shows a collimated or focused reference laser beam 71 asreference element for the x-carriage 22. Thereby, for sake ofsimplicity, only the bridge 20 (X-beam) with the X-carriage 22 and thez-beam 24 (which is movable relative to the x-carriage 22 inZ-direction) are depicted as CMM-components.

The laser source 75 is installed on one side of the X-beam 20(CMM-bridge), so that the laser beam 71 behaves in X-direction andparallel to the bridge 20. The reference beam represents the referencepath. Exemplarily, the laser source 75 may be designed as a laser diodewith collimation optics.

A displacement sensor 9 is assigned to the reference laser beam 71. Thedisplacement sensor 9 is attached to the X-carriage 22 in such a waythat it faces towards the laser source 75. The displacement sensor 9 isdesigned as a photosensitive detector element being built for measuringan impinging position of the reference laser beam 71. For example, thephotosensitive detector element may be designed as CCD-martix array,CMOS-matrix array, PSD-sensor (position sensitive device) or quadrantdetector.

The sensed impinging position of the laser beam 71 indicatestranslational displacements of the X-carriage 22 in directionsorthogonal to the X-direction (particularly translational displacementsin Y- and Z-directions). In case of a collimated beam as reference, thesection width of the beam may be defined and, for precisely determiningan impinging position of the beam, a center or midpoint of the projectedbeam spot may be determined as exact impinging position. According to aspecial embodiment, also the shape of the projected beam spot onto thesensor may be determined, analysed and used for deriving a pitchingand/or yawing error of the X-carriage. For example, an ellipsoidalprojection of the reference beam wherein the semi-major axis is alignedin Z-direction indicates a pitching error and an ellipsoidal projectionof the reference beam wherein the semi-major axis is aligned inY-direction indicates a yawing error of the X-carriage.

The indicated and determined translational and/or rotationaldisplacements, hence, can be used by the calculation unit of the CMM fordetermining the spatial coordinates of a measurement point approached bythe probe head.

In FIG. 4, a close-up front view of the linear X-drive mechanism of aCMM according to the invention is represented. Similarly to FIG. 3, acollimated or focused reference laser beam 71 is used as opticalreference element.

The laser beam 71 behaves in X-direction and parallel to the bridge 20(X-beam). The reference beam 71 represents the reference path.

The two displacement sensors 9 a,9 b being placed on top of theX-carriage 22 (depicted with broken lines) measure displacements of theX-carriage 22 with respect to the reference element 71. As shown in moredetail in FIG. 5, the displacement sensors 9 a,9 b may comprise a beamsplitter 91 a,91 b for coupling out a part of the reference beam 71 anddirecting it onto a photosensitive detector element 92 a,92 b. Therein,the photosensitive detector 92 a, 92 b is built for determining animpinging position of the coupled out and reflected beam. For example,the photosensitive detector 92 a,92 b may be designed as CCD-matrixarray, CMOS-matrix array, PSD-sensor or quadrant detector. Again, theimpinging point of the reflected part of the reference beam onto thedetector is used for determining translational displacements of thecarriage 22 in the Y-Z-plane. By considering the impinging points of thereflected beams detected by both two displacement sensors 9 a,9 b, alsorotational displacements of the carriage 22 (i.e. yawing and pitchingerror) can be determined (particularly by a differential evaluation ofthe outputs of both sensors).

Alternatively to the above explained embodiment of the sensors (whichcomprise a beam splitter for coupling out a part of the reference beamand directing it onto a photosensitive detector element), a transparentphoto-sensitive detector element for determining an impinging positionof the reference beam may also be used.

FIG. 6 shows a side view of a bridge-type CMM 1 with a mechanicalreference element 72, wherein displacement sensors 9 are assigned to thereference element 72 in order to measure distances to the table-surface61.

The frame structure of the CMM is subject to carrying and portativeforces. However, according to the invention, the reference element 72 isattached to the frame structure in such a way, that substantially nocarrying or portative forces effect onto the reference element 72(respectively the reference path generated by the reference element).

For example, the mechanical reference element 72 is mounted only on itsupper end onto a side of the X-beam 20 (bridge) of the CMM. Thereference element 72 extends along the leg 18 and over a part of theedge of the table 6 in such a way, that it is distanced apart (a smalldistance) from the edge of the table 6.

The reference sensors 9 can be designed as optical or capacitivedistance sensors and mounted onto the reference element 72 in such a waythat distances from the reference element 72 (i.e. from the positions ofthe sensors) to the upper and side surface 61 of the table 6 can bemeasured.

These distances indicate translational and/or rotational displacementsin the linear Y-drive mechanism (e.g. translational displacements in X-and Z-direction and rotational displacements like a pitching, rollingand yawing error).

For example, three distance sensors can be mounted to the referenceelement in order to measure distances to the upper surface of the table(for indicating a translational displacement in Z-direction and apitching error) and two distance sensors can be mounted to the referenceelement in order to measure two distances to the side surface of thetable (for indicating a translational displacement in X-direction and ayawing error). A rolling error can be derived from a combination of theoutputs of the sensors.

As the reference element 72 is mounted directly to the side of theX-beam 20 (bridge), the position of the X-beam 20 (bridge) can directlybe referenced relative to the table 6 of the CMM, so that themeasurements are independent of actual load conditions of the CMM-legs18 (or so that the actual load conditions of the CMM-legs 18 can bedetermined and considered for deriving the measurement position of theprobe head).

The sensed displacements, hence, may be used in order to correct thecalculation for the position of the probe relative to the base.

FIG. 7 shows a side view of a bridge-type CMM with a mechanicalreference element 72. The reference element 72 is embodied as referencerod and used in order to sense weaknesses and deformations in the framestructure (i.e. the leg 18) of the CMM. Such weaknesses and deformationsin the leg 18 of the CMM-frame structure may for example be caused byload, vibration, dynamic effects, temperature variations, pressurevariations, aging, humidity variations, etc.

In the shown embodiment, the reference rod—as reference element 72—isfixedly attached to the foot 28 (Y-carriage) of the CMM-frame structureand extends along the leg 18 until one side of the bridge 20. Twodisplacement sensors 9 a,9 b are mounted to the bridge 20, facingtowards the loose end of the reference rod 72 (i.e. the free end whichis not fixedly attached to the frame structure).

The displacement sensors 9 a,9 b measure a displacement (i.e. adeformation like e.g. a bending) of the frame structure in that part(i.e. the leg 18), which is spanned by the reference rod 72. Therefore,the positions of the displacement sensors 9 a,9 b—being mounted to thebridge—are referenced with respect to the upper end of the unloadedreference rod 72.

Exemplarily, a deformation of the leg 18 will cause a change in thedistances from the displacement sensors 9 a,9 b to defined locations onthe upper end of the reference rod 72. That distances can be measured bythe displacement sensors 9 a,9 b (being embodied e.g. as optical orcapacitive distance sensors) and used in order to calculative compensatefor the sensed deformation of the CMM-leg 18 when determining themeasuring position.

By using mechanical and/or optical reference elements in the sense ofthe invention, a high accuracy in the measurements can be ensured,although the load carrying components of the CMM may have comparativelylow dimensional stability and low solid gauge. Even in cases that thereference elements themselves loose their required dimensional stabilityfor accurate measurements (i.e. caused by aging effects, etc.), they maybe exchanged more easily and under less effort compared to an exchangeof aged parts of the frame structure (i.e. the leg). Hence, the lifetimeof a CMM may be extended according to the invention, as even in cases ofaging effects, the reference elements can be renewed and exchangedcomparatively easily and, thus, accurate measurements may continuativelybe ensured—by referencing parts of the weak frame structure with respectto reference elements and compensating for the weaknesses according tothe invention.

FIG. 8 shows an embodiment, wherein the features of FIGS. 6 and 7 arecombined.

Similar to FIG. 6, a side view of a bridge-type CMM 1 with a mechanicalreference element 72 is depicted. The mechanical reference element 72 ismounted only on its upper end onto a side of the X-beam 20 (bridge) ofthe CMM. The reference element 72 extends along the leg 18 and over apart of the edge of the table 6 in such a way, that it is distancedapart (a small distance) from the edge of the table 6 and from the foot28. Exemplarily, four displacement sensors 9 a are installed onto theloose, lower end of the reference element 72 in order to measuredistances to the table-surface 61 and, additionally, similar to theembodiment of FIG. 7, three displacement sensors 9 b are installed ontothe loose, lower end of the reference element 72 in order to measuredistances to defined locations of the foot 28. These distances to thefoot 28 indicate deformations of the leg component 18.

The reference sensors 9 a,9 b can be designed as optical or capacitivedistance sensors and mounted onto the reference element 72 in such a waythat distances from the reference element 72 (i.e. from the positions ofthe sensors) to the upper and side surface 61 of the table 6—as well assurfaces of the foot 28—can be measured.

The indicated and determined displacements and deformations, hence, canbe used in order to correct the calculation for the position of theprobe relative to the base.

Applying such a setup—according to the embodiment of FIG. 8—allows forimproved compensation of weaknesses in the bearings/joints of the CMM aswell as for compensation of weaknesses in the load-carrying framestructure of the CMM (i.e. bending in the leg).

FIG. 9 and FIG. 10 shows a front respectively a side view of agantry-type CMM 1—according to the invention—with a second, mechanicalreference element 72 and a first, optical reference element 71.

As shown, a gantry-type CMM does not have movable legs/feet between theY/W- and X-axis 20 (or at least they are very short). The X-beam 20 isdirectly (i.e. without or with only very short feet) supported formovement by bearings in the Y direction along Y- and W-beams 50,52 whichare carried by four pillars 54,56,58. The pillars are rigidly mounted ona measurement table, as known per se to a skilled person. Furthermore,there exist linear measuring instruments in the X-, Y/W- and Z-drivemechanisms (for the sake of simplicity only shown in the X- andY/W-drive mechanisms).

The second reference element 72 is attached to one side of the X-beam 20and extends until the edge of the table. Similarly to FIG. 6,displacement sensors 9 b mounted to the reference element 72 measuredistances to defined locations of the upper and side surface of thetable. These distances indicate displacements of the X-beam 20 causede.g. by weaknesses in the Y-bearings, which support for movement of thebridge 20 (X-beam) relative to the Y- and W-beams 50,52.

Furthermore, analogous to FIG. 4, a collimated or focused referencelaser beam is used as first reference element 71. Therefore, a lasersource 75 is mounted to one end of the X-beam 20 for projecting thereference laser beam. The laser beam behaves in X-direction and parallelto the bridge 20 (X-beam). The reference beam represents the referencepath.

Two displacement sensors 9 a are placed on top of the X-carriage 22, thedisplacement sensors measuring displacements of the X-carriage 22 withrespect to the reference beam 71.

These displacements of the X-carriage 22 may e.g. be caused byweaknesses in the X-bearings, which support for movement of theX-carriage 22 relative to the X-beam 20.

FIG. 11 shows a front view of a bridge-type CMM with an opticalreference element for indicating a bending of the bridge.

The optical reference element is embodied as a collimated or focusedreference laser beam 71. The laser source 75 is installed on one side ofthe X-beam 20 (CMM-bridge), so that the laser beam 71 behaves inX-direction and parallel to the bridge 20. The reference beam 71represents the reference path.

A displacement sensor 9 is assigned to the reference laser beam 71.Thereby, the displacement sensor 9 is mounted to the other side of theX-beam 20 (CMM-bridge) in such a way that it faces towards the lasersource 75. The displacement sensor 9 is designed as a photosensitivedetector element being built for measuring an impinging position of thereference laser beam. For example, the photosensitive detector elementmay be designed as CCD-martix array, CMOS-matrix array, PSD-sensor(position sensitive device) or quadrant detector.

The sensed impinging position of the laser beam 71 indicates adeformation of the X-beam 20. In case of a collimated beam as reference,the section width of the beam may be defined and, for preciselydetermining an impinging position of the beam, a center or midpoint ofthe projected beam spot may be determined as exact impinging position.According to a special embodiment, also the shape of the projected beamspot onto the sensor may be determined, analysed and used for deriving adeformation of the X-beam 20.

The impinging position of the reference beam 71 (i.e. the relativeposition of the sensor with respect to the reference beam) indicates thecorresponding dimensional condition (regarding a deflection or bending)of the X-beam 20.

The indicated and derived deformation or deflection of the X-beam 20,hence, can be used by the calculation unit of the CMM for determiningthe spatial coordinates of a measurement point approached by the probehead.

FIG. 12 shows—similarly to FIG. 11—a front view of a bridge-type CMM 1with another alternative embodiment of an optical reference element 71for indicating a deformation, particularly a bending, of the bridge 20.

According the embodiment of FIG. 12, the laser source 75 for thereference laser beam 72 and the sensor 9 are mounted on the same end ofthe X-beam 20 and on the other end, there is installed a reflector prism76 for retroreflecting the reference beam 71. The deformations(particularly bending) of the X-beam 20 may be carried out in ananalogous way as described in connection with FIG. 11. However, theinstallation of the laser source 75 and the sensor 9 on one and the sameend of the X-beam 20 may provide advantages regarding control and powersupply of the laser source 75 and the sensor 9 (connection with thecontrol system).

FIG. 13 shows another embodiment of a bridge-type CMM 1 with an opticalreference element 71 for indicating a bending of the bridge 20,similarly to the embodiment as depicted in FIG. 11.

Additionally to the laser source 75 on one end of the X-beam 20 and afirst optical sensor 9 a on the other end of the X-beam 20 (analogouslyto FIG. 11), a second optical sensor 9 b is installed for measuring adisplacement of a position of the X-beam 20 being situated about in themiddle of the beam. Exemplarily, the second sensor can be installed ontoa mechanical sensor holder 76 which itself is fixed to the lower surfaceof the X-beam 20 (in order not to disable movement of the X-carriage22). The sensor 9 b indicates a deformation of the X-beam 20 relative tothe reference beam 71.

For example, the second displacement sensors 9 b may comprise a beamsplitter for coupling out a just part of the reference beam 71 anddirecting it onto a photosensitive detector element. Therein, thephoto-sensitive detector may be built for determining an impingingposition of the coupled out and reflected beam, as for example alreadyshown in connection with FIG. 5.

The impinging point of the reflected part of the reference beam onto thedetector is used for determining a displacement of a position in themiddle of the X-beam 20 (where the sensor holder 76 is mounted to theX-beam 20) with respect to the reference beam 71. Therefore, adeformation of the X-beam 20 (i.e. a bending) can be derived from theimpinging point and used for compensating the deformation error whencalculating the spatial coordinates of a measurement point approached bythe probe head.

Alternatively to the above described embodiment of the sensor 9 b (i.e.comprising a beam splitter for coupling out a part of the reference beamand directing it onto a photosensitive detector element), also atransparent photosensitive detector element for determining an impingingposition of the reference beam may be used as the second displacementsensor.

As described above in connection with the bridge-type CMM, also inconnection with other types of CMMs—e.g. L-bridge-type,horizontal-arm-type, cantilever-type, gantry-type, etc.—dynamicmovements and geometrical errors in the frame structure (weaknesses inthe frame material) and/or error displacements between the movingmembers and the guides of the X-, Y/W- and Z-drives may accordingly besensed and compensated for using a reference element (being mounted tothe frame structure in an substantially unloaded way) and displacementsensors.

Hence, although the invention is illustrated above, partly withreference to some preferred embodiments, it must be understood thatnumerous modifications and combinations of different features of theembodiments can be made. All of these modifications lie within the scopeof the appended claims.

Some above described embodiments according to the invention areexemplarily shown only for one linear drive mechanism or only for onepart of the CMM. However, of course, the inventive approaches may alsoor alternatively be applied for each of the other drive mechanisms andother parts of the CMM. Furthermore, the inventive concept may beapplied for a CMM being designed as parallel-kinematics machine as wellas for a CMM having linear or serial kinematics, as shown in thefigures.

What is claimed is:
 1. A coordinate measuring machine for determinationof at least one spatial coordinate of a measurement point on an objectto be measured, comprising: a base; a probe head for approaching themeasurement point, a frame structure for linking the probe head to thebase, the frame structure including: at least a first and a second framecomponent; and at least one linear drive mechanism moveably linking thefirst and the second frame components, for provision of movability ofthe probe head relative to the base in a first direction, wherein the atleast one linear drive mechanism comprises: a linear guide in the firstdirection; a movable member being supported for movement along the guideby bearings; and a linear measuring instrument for determination of afirst drive position of the movable member in the first direction,wherein: at least a first optical reference element designed as anoptical reference beam for providing a first reference path, wherein thereference beam extends along the guide of the linear drive mechanism sothat the reference path is parallel to the first direction; and at leasta first and a second displacement sensor are assigned to the referencebeam, the reference beam and the displacement sensors being designed andarranged in such a way, that two or more distances from definedpositions on the movable member to the reference beam and/or impingingpositions of the reference beam are measurable by the displacementsensors, wherein the distances respectively impinging positions indicatetranslational and rotational displacements of the movable member fromthe ordinary bearing position, wherein at least the first displacementsensor being built so as to partly transmit the optical reference beamand at least the second displacement sensor being assigned to atransmitted part of the optical reference beam and the displacementsensors being built as photosensitive detector elements being built formeasuring distances to the reference beam and/or an impinging positionof the reference beam indicative of the displacement of the movablemember relative to the first reference path in a direction perpendicularto the first direction.
 2. The coordinate measuring machine as claimedin claim 1, wherein the displacement sensors further comprise a beamsplitter for coupling out at least a part of the reference beam anddirecting it onto the photosensitive detector elements.
 3. Thecoordinate measuring machine as claimed in claim 2, wherein thephotosensitive detector elements are built as CCD-array, CMOS-array, PSDor quadrant detectors.
 4. The coordinate measuring machine as claimed inclaim 1, wherein the coordinate measuring machine comprises acalculation unit for determination of the space coordinate as a functionof at least: the first drive position and the translational and/orrotational displacements of the movable member from the ordinary bearingposition.
 5. The coordinate measuring machine as claimed in claim 1,wherein: the linear guide is provided on or by the first frame componentand the movable member is provided on or by the second frame component,and a laser source providing for the reference beam is installed on thefirst frame component and the displacement sensors are attached to thesecond frame component in such a way that they face towards the lasersource.
 6. The coordinate measuring machine as claimed in claim 5,wherein the laser source is designed as a laser diode with collimationoptics.
 7. The coordinate measuring machine as claimed in claim 1,wherein the frame structure comprises: four frame components and threelinear drive mechanisms moveably linking the four frame components, forprovision of movability of the probe head relative to the base in afirst, a second and a third direction, each linear drive mechanismincluding: a linear guide in the first, the second respectively thethird direction; a movable member being supported for movement along theguide by bearings; a linear measuring instrument for determination ofthe first, a second or a third drive position, respectively, of themovable member in the first, the second or the third direction,respectively; wherein, the coordinate measuring machine comprises acalculation unit for determination of the space coordinate as a functionof at least the first, the second and the third drive position and thetranslational and/or rotational displacements of the movable membersfrom its respective ordinary bearing positions indicated by thedisplacement sensor.
 8. The coordinate measuring machine as claimed inclaim 1, wherein: a second or more reference elements being arranged onthe frame structure, each for providing a substantially unloadedreference path along a part of the frame structure; and at least onedisplacement sensor are assigned to each of the reference elements, thereference elements and the displacement sensors being designed andarranged in such a way, that displacements and/or deformations of theframe structure are measurable relative to the respective referencepaths.
 9. The coordinate measuring machine as claimed in claim 1,wherein: a second or more reference elements being arranged on the framestructure, each for providing a substantially unloaded reference pathalong a part of the frame structure; and two to five displacementsensors are assigned to each of the reference elements, the referenceelements and the displacement sensors being designed and arranged insuch a way, that displacements and/or deformations of the framestructure are measurable relative to the respective reference paths. 10.The coordinate measuring machine as claimed claim 1, wherein the secondor more reference elements each are designed as: a mechanical referenceframe extending along the respective part of the frame structure,wherein the reference frame being fastened fixedly to the framestructure in a substantially unloaded way, in particular wherein thereference frame is fastened only on one of its ends to the framestructure; or a further optical reference beam, in particular acollimated or focused laser beam, which extends along the respectivepart of the frame structure.
 11. The coordinate measuring machine asclaimed claim 1, wherein the second or more reference elements each aredesigned as: a collimated or focused laser beam which extends along therespective part of the frame structure.
 12. The coordinate measuringmachine as claimed in claim 1, wherein the coordinate measuring machineis designed as parallel-kinematics machine or as machine having linearor serial kinematics, particularly wherein the coordinate measuringmachine is designed as claimed in one of the following styles:bridge-type, L-bridge-type, horizontal-arm-type, cantilever-type, organtry-type.
 13. The coordinate measuring machine as claimed claim 1,wherein a contact probe or a non-contact probe is arranged on the probehead; and/or the base comprises a table with a granite surface plate forsupporting the object to be measured.
 14. The coordinate measuringmachine as claimed claim 1, wherein a scanning or touch trigger probe isarranged on the probe head.
 15. The coordinate measuring machine asclaimed claim 1, wherein an optical, capacitance or inductance probe, isarranged on the probe head.
 16. A coordinate measuring machine fordetermination of at least one spatial coordinate of a measurement pointon an object to be measured, comprising: a base; a probe head forapproaching the measurement point, a frame structure for linking theprobe head to the base, the frame structure including: at least a firstand a second frame component; and at least one linear drive mechanismmoveably linking the first and the second frame components, forprovision of movability of the probe head relative to the base in afirst direction, wherein the at least one linear drive mechanismcomprises: a linear guide in the first direction; a movable member beingsupported for movement along the guide by bearings; and a linearmeasuring instrument for determination of a first drive position of themovable member in the first direction, wherein: at least a first opticalreference element designed as an optical reference beam for providing afirst reference path, wherein the reference beam extends along the guideof the linear drive mechanism so that the reference path is parallel tothe first direction; and three to five displacement sensors are assignedto the reference beam, the reference beam and the displacement sensorsbeing designed and arranged in such a way, that two or more distancesfrom defined positions on the movable member to the reference beamand/or impinging positions of the reference beam are measurable by thedisplacement sensors, wherein the distances respectively impingingpositions indicate translational and rotational displacements of themovable member from the ordinary bearing position, wherein at least thefirst displacement sensor being built so as to partly transmit theoptical reference beam and at least the second displacement sensor beingassigned to a transmitted part of the optical reference beam and thedisplacement sensors being built as photosensitive detector elementsbeing built for measuring distances to the reference beam and/or animpinging position of the reference beam indicative of the displacementof the movable member relative to the first reference path in adirection perpendicular to the first direction.
 17. A method ofcompensating errors in a coordinate measuring machine which determinesat least one spatial coordinate of a measurement point on an object tobe measured, the coordinate measuring machine including: a base; a probehead for approaching the measurement point; and a frame structure forlinking the probe head to the base, wherein the frame structurecomprises at least one linear drive mechanism for provision ofmovability of the probe head relative to the base in a first direction,wherein the at least one linear drive mechanism includes: a linear guidein the first direction; a movable member being supported for movementalong the guide by bearings; and a linear measuring instrument fordetermination of a first drive position of the movable member in thefirst direction, wherein the method comprises: providing a firstreference path parallel to the first direction by generating an opticalreference beam extending along the guide of the linear drive mechanism;measuring at least two displacements of the movable member relative tothe first reference path so that the displacements are indicative of atranslational and rotational displacement of the movable member from anordinary bearing position, wherein measuring the displacements isprovided by displacement sensors being built as photosensitive detectorelements being built for measuring distances to the reference beamand/or an impinging position of the reference beam indicative of thedisplacement of the movable member relative to the first reference pathin a direction perpendicular to the first direction and at least a firstdisplacement sensor being partly transmissible for the optical referencebeam and at least a second displacement sensor being assigned to atransmitted part of the optical reference beam; and compensating for theerrors by using the determined actual at least one displacement.
 18. Themethod as claimed in claim 17, wherein the errors compensated forinclude weaknesses in the bearing of the linear drive mechanism.